1. Field Of The Invention
The invention relates to a method for the preparation of very finely particulate .alpha.-aluminum oxide powder, in particular for the production of high-performance ceramics, from aluminum hydroxide (gibbsite or hydrargillite) or aluminum oxide which has been calcined at low temperature.
2. Background Art
.alpha.-Aluminum oxide powder has been prepared for many decades by the calcination of aluminum hydroxide. The calcination temperature in this process is usually above 1300.degree. C. As the calcination temperature rises, the proportion of o-aluminum oxide increases, but at the same time so does the size of the primary crystals. The specific surface of the product, on the other hand, decreases with increasing calcination temperature. Powders to be used for the production of high-performance ceramics preferably have particle sizes below 1 .mu.m. Ordinary calcination, however, produces agglomerates which are composed of primary crystals with sizes of the order of from 2 to 10 .mu.m, and it is therefore necessary subsequently to mill the powder. Not only are the agglomerates broken down in this process, but the primary crystals are also reduced in size. Drawbacks of this procedure are the high expenditure of time and energy for milling, and the need for specific milling processes, not only so as to achieve the final size desired, but also to avoid contamination by abrasion from the milling device. Finally, the properties of the .alpha.-aluminum oxide powders thus obtainable are also adversely affected by their wide particle size distribution and the irregular particle shape. In contrast, the achievement of optimum results in high-performance ceramics requires powders having narrow particle size distributions and compact (equiaxed) particles; such powders, in spite of the compact particles, have a large specific surface. Such powders have been produced for some years from boehmite (AlOOH) according to the "seeded gel" process. In this case, boehmite is wet-mixed with .alpha.-Al.sub.2 O.sub.3 nuclei, gelled, dried, calcined and wet-disagglomerated. A drawback of the process is the high price of the starting material boehmite.